1. Field of the Invention
This invention relates to an electronic apparatus, and more particularly, to an electronic apparatus which is capable of mounting extended IC cards.
2. Description of the Related Art
Examples of electronic apparatus which are capable of mounting extended IC cards include electronic typewriters which edit and print out documents, or the like. IC cards which can be mounted on such typewriters include RAM cards for the extension of text RAM's (used for preparing a document containing a large number of sentences), spelling corrector cards for performing a spelling check of English words, ROM cards for storing patterns of special characters, and the like. Heretofore, IC cards of different kinds differed in size depending on the difference in function, the number of constituent IC chips or the like, and heretofore each kind of IC card has been adapted to be connected to a predetermined exclusive connector on the body of an electronic apparatus.
When the mounting position of an electronic apparatus is thus fixed for each kind of IC card, the general applicability of the electronic apparatus is limited. Recently, owing to progress in semiconductor technology, it has become possible to provide IC cards having an entirely identical shape and size even for different kinds of IC cards, and to provide plural connectors on the body of the electronic apparatus in an identical shape and size for common use. In other words, the signals lines, each of which is input to each connector for extending the body of the electronic apparatus, all have an identical layout.
This arrangement is illustrated in FIG. 7. In FIG. 7, the connection terminal sides of IC cards 71 and 72 are connected to chip-selection signal lines 76 and 77, respectively. Although, in FIG. 7, the IC card 71 is connected to a connector 73, and the IC card 72 is connected to a connector 74, the combination may also be reversed.
Now, suppose that a CPU 81 accesses the IC card 71. The corresponding address is then output on an address bus 79 for that purpose. The address bus 79 is also connected to an address decoder circuit 78 as shown in FIG. 7. The chip-selection signal line 76 is activated ("1" in the case of positive logic, and "0" in the case of negative logic) by the address decoder circuit 78. The CPU 81 thereby becomes capable of delivering and receiving data to and from the IC card 71 connected to the connector 73 or 74 via a data bus 80.
However, when, for example, in the configuration of FIG. 7, the IC card 72 is exactly the same kind of card as the IC card 71, both cards become simultaneously accessible. When the CPU 81 tries to read data from one of the IC cards, data are almost simultaneously output from each IC card, and thus data from both cards conflict with each other. That is, the electronic apparatus performs an unauthorized operation. This results in a very dangerous state.